Borrelia miyamotoi Transmission: Larval Deer Ticks Can Infect

Larval deer ticks are often overlooked
Borrelia miyamotoi can be passed through tick eggs
Even newly hatched larvae may transmit infection

Borrelia miyamotoi transmission changes how we think about larval deer ticks.

In Lyme disease, larval ticks are typically not infected with Borrelia burgdorferi because the bacteria are not usually passed through tick eggs.

But Borrelia miyamotoi is different.

Researchers have shown that infected adult ticks can pass B. miyamotoi to their offspring through transovarial transmission, meaning larval ticks may already carry infection when they hatch.

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What Borrelia miyamotoi Transovarial Transmission Means

According to investigators from the Centers for Disease Control and Prevention, larval ticks can already be infected with Borrelia miyamotoi after they hatch.

This occurs through transovarial transmission, a process in which an infected adult female tick passes the bacteria to offspring through infected eggs.

This is clinically important because larval ticks are often assumed to be harmless compared with nymphal ticks.

That assumption may not hold for B. miyamotoi.

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CDC Study on Larval Tick Transmission

Researchers infected experimental mice with Borrelia miyamotoi, the relapsing fever bacterium.

They found that “minimal or partial blood meals by single-feeding transovarially B. miyamotoi-infected larvae also resulted in approximately half of experimental hosts developing infections detectable by examination of blood for presence of B. miyamotoi DNA,” writes Breuner from the CDC’s National Center for Emerging and Zoonotic Infectious Diseases.

This finding challenges the assumption that larval ticks are always low risk.

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Can Larval Ticks Transmit Lyme Disease?

Many people ask whether larval ticks can transmit Lyme disease.

Most larval deer ticks are not considered an important source of Lyme disease because Borrelia burgdorferi, the bacterium that causes Lyme disease, is generally not passed from infected female ticks to their eggs.

However, larval ticks can transmit Borrelia miyamotoi, a relapsing fever spirochete that is transmitted through transovarial transmission.

This means infected female ticks can pass the infection directly to their offspring.

As a result, larval ticks may carry Borrelia miyamotoi even though they are unlikely to transmit Lyme disease itself.

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Why Larval Deer Ticks Pose a Unique Risk

Larval ticks are extremely small and easy to miss.

They are often active in July and August, when many people are outdoors and may be less vigilant about tick protection.

Public health messaging has traditionally emphasized nymphal ticks in spring and early summer.

However, this study suggests that late-summer exposure may also carry risk when larval ticks are infected with B. miyamotoi.

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How This Differs From Lyme Disease

In Lyme disease, larval ticks are generally not considered an important source of infection because Borrelia burgdorferi is not typically transmitted through eggs.

By contrast, B. miyamotoi can be passed from infected female ticks to offspring.

This distinction matters for prevention, diagnosis, and patient education.

Patients bitten by larval ticks may still need monitoring for symptoms of tick-borne coinfections and relapsing fever-type illness.

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Public Health Implications

The authors suggested that public health officials consider revising tick-bite prevention messaging to include larval ticks and their seasonal activity.

This is especially important in July and August, when larval ticks may be active and people may mistakenly assume the highest-risk season has passed.

Prevention messages should include daily tick checks, protective clothing, repellents, and awareness that very small ticks can still matter clinically.

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Clinical Perspective

Most clinicians focus on nymphal deer ticks because they are the primary source of Lyme disease transmission.

However, Borrelia miyamotoi behaves differently from Borrelia burgdorferi. Because infected female ticks can pass the organism directly to their offspring, larval ticks may represent a clinically relevant source of infection.

This distinction may be important when evaluating patients who develop symptoms after late-summer tick exposure.

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Frequently Asked Questions

Can larval ticks transmit disease?

Yes. Although larval ticks are less important than nymphs for Lyme disease transmission, studies have shown that larval deer ticks can transmit Borrelia miyamotoi through transovarial transmission.

Do larval ticks carry Lyme disease?

Most larval ticks are not considered an important source of Lyme disease because Borrelia burgdorferi is generally not passed through tick eggs.

Can larval ticks bite humans?

Yes. Larval deer ticks can attach to and feed on humans, although their tiny size often makes them difficult to detect.

When are larval ticks most active?

Larval deer ticks are most active during July and August in many endemic regions.

Are larval ticks dangerous?

Most larval ticks are considered lower risk for Lyme disease than nymphs. However, studies have shown that larval deer ticks can transmit Borrelia miyamotoi, meaning they should not automatically be considered harmless.

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Clinical Takeaway

Larval deer ticks can transmit Borrelia miyamotoi when they acquire infection through transovarial transmission.

This expands the clinical importance of larval tick bites and late-summer tick exposure.

Patients and clinicians should not assume that larval ticks are harmless, particularly when symptoms follow a summer tick bite.

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Related Articles

• Borrelia miyamotoi
• Do Tick Larvae Bite Humans?
• Study Demonstrates Larval Ticks May Be a Threat
• Prevention of Lyme Disease

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References

1. Breuner NE, Hojgaard A, Replogle AJ, Boegler KA, Eisen L. Transmission of the relapsing fever spirochete, Borrelia miyamotoi, by single transovarially-infected larval Ixodes scapularis ticks. Ticks Tick Borne Dis. 2018.

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Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.

Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention